how to track pay-per-minute?

I saw some private channels offering pay-per-minute content and wondering how this is done. Does the roku app track how long the video player stays open? Or does the media server detect for how long streams are requested?
In the past I tried to generate reports showing how long users watched but wasn't able to get any kind of accurate measure, so I am really curious how they can do this with any accuracy.
tvmia,
Joe